Ferro-prussiate printing



' Sept. 17, 1935. H, D MURRAY 2,014,692

FERRO-PRUS S IATE PRINTING Filed March 3. 1934 OBJECT UNEXPOSED PAPER ATED WITH A soLunaLs FERRIC SALT AND, F? SOLUBLE FERRICYH NI DE EXPOSED PAPER BR IGHT YELLOW BYTREATMENT wrm BYTREATMENT wrrH NICKEL SALTS ZINC SALTS BY TREATMENT WITH BY REATMENT WITH CADMIUM SALTS MANGANESE SALTS BLUE DARK GREEN FROM FIG-3. av TREATMENT wrrn BY TREATMENT wrn-l COBALT SALTS NlCKEL SALT-S COMPLEMENTARY YELLOW DYESFUFF INVEHTOR' i BRIGHT YELLOW Patented Sept. 17, 1935 London, England,

assignor of two-thirds to Douglas Arthur Spencer, London, England, and Norton & Gregory Limited, London, England, a

British company Application March 3, i934, Serial No. 713,944

In Great Britain March 10, 1933 8 Claims. (Cl. 95-6) This invention is for improvements in and relatlng toferro-prussiate. printing, and comprises an improved sensitized basev and an improved print and also their process oi production.

It is well known to prepare a light-sensitive base, usually paper, by coating with a solution containing a ferric salt and a ierricyanide and drying. On exposure of such a base to light behind a design, the ferric salt on the portions uns screened from the light is reduced to a ferrous salt, which is capable of reacting with the ferricyanide to produce a blue coloum The prints are fixed by washing the base in water, whereby the soluble salts are removed and so. further action w of light prevented. This process is commonly known as the ierro-prussiate', or blue-print,

process. 7 I

Hereinafter, a base which been treated with light-sensltive reagents in the manner Just no dwcribed will be referred to as a ferro-prussiate paper and a print therefrom as a ferro-prussiate print. The word "paper" is intended to include,

as well as paper itself, any other suitable base.

According to this invention, a ferro-prussiate up printis produced by treating a ferro-prussiate paper, after exposure, with a fixing agent for one or other of the primary reagents with which the paper has been coated, for the purpose or directly developing the picture to a light-re= so clstant print. 1 Thus, it the exposed paper be treated, for ex ample, sponged over, with a dilute solution oi a weak alkali, such'as bonsx or an alkali pe1'=- hereto, the negative imageis fixed in the sense to that it may be further exposed'ftoilight without turning blue. Orthophenanthrollneis an example of another type of compound which, poo searing the property of forming a non-reactive co-ordinated compound with iron, may be emiid ployed. This method of fixing the residual iron salt has, however, the disadvantages oi reducing the colour of the blue background and tending to make the lines of the image run. substances which fix only the iron salt in the exposed print 45 do not appear to have the power of preserving the blue background against the bleaching action of light over long exposure, and it is preferred, therefore, to utilize a developer whose primary function is to fix the unchangedflerricy- 60 snide; and this developer may comprise an agent .having the property of fixing the residual iron salt, as will be presently explained.

According to this preferred formof the invention, the ferro-prusslate paper is treated sub-- 6 sequent to exposure to light with a ferriyanide lnsolubilizer or fixing agent, that is to say, an agentcapable of reacting with the soluble ferricyanide to give an insoluble derivative. Exampies of.s uitable agents of the kind-are, in order of preference, solutions of salts of nickel, zinc, manganese, cadmium and cobalt. Soluble organic bodies, capable of giving insoluble compounds with ferricyanides, may be employed; but a not uncommon drawback to such reagents is that they frequently impart undesir- 10 able colours to, the print particularly on expos= ure tolight and air. For example, a lerro-prusslate paper treated with a developing solution containing the basic dyestufi Auramln'e, Colour hide-x655. and a mild oxidizing agent, is read- 35 dered stable to exposure to llghtif the exposure be not too prolonged. For the most eflective results, the ierricyanide insolubiliaer shouldbe employed in as high a concentration as is consistent with avoidance of crystallization upon the surface of the print as the developing solution dries. Thus, for application in a thin uniform film by mechanical means, solutions, oi the following concentrations may be used: c

' Percent Nickelsulphate 15-20 Manganese sulphate 20-30 Zinc Sulphate -40 For application by hand, these solutions should so be diluted in the proportion of 3 volumes of water per 10 volumes of solution in order to allow for the greater quantity of liquid received by the paper. Otherwisethe salts may crystallize 85 and'form a bloom.

The respective colours of the lines produced by the aforesaid metal salt developers are as follows:

Nickel salts Bright yellow Zinc salts. Pale yellow 40 Manganese salts Salmon pink I @admium. salts Pale yellow Cobalt salts Brick: red. 1

The developing solution may also. comprise a 66 towards light.

wetting agent, that is, a substance which lowers the surface tension of the solution, thereby increasing the tendency of the liquid to spread and to penetrate into the material of the base.

I have found it to be distinctly advantageous that the term-prussiate print should comprise an oxidizing agent, which may be conveniently applied as a component of the developing solution. Such an. agent appears to fulfill at least two functions; darkening the colour of the background and increasing the stability of the print My experience has also shown that the best effect is obtained by the association of a strong or quick-acting oxidizing agent, such as a persulphate, with a comparatively mild or slow-acting oxidizer, such as a chlorate. A chlorate may also be used alone.

The stronger oxidizer need only be present in the developing solution in sufficient concentration to bring'out the full depth of colour of the background. For example, a suitable concentration for ammonium persulphate is 2 per cent. The' slow-acting oxidizer should be employed in the highest possible concentration consistent with avoidance of crystallization. For example, a solution for mechanical application may contain about 10-20 per cent of sodium chlorate.

I have further found that in the development of ferro-prussiate prints with a ferricyanideinsolubilizer it is desirable carefully to adjust the acidity of the base and of the developing solution so that, onthe one hand, there shall not be present enough free acid to dissolve any of the insoluble ferricyanide produced and so give rise to free ferricyanide ions capable of causing discolouration in the originally screened parts of the base on re-exposure of the latter to light, and that, on the other hand, the base shall been the acid side of neutrality in order that the Prussian blue of the background may not be bleached. Mild alkalies have the effect of converting Prussian blue into a white leuco-compound, especially on exposure to air, and hence are not to be recommended by themselves for adJustment of the acidity. Such adjustment is preferably made by applying the ferricyanide-insolubilizer in a buffered solution.

It appears probable that the presence in the developer of a buffer reagent permits of the formation on the prussiate paper of insolubie basic salts of iron in place of the original soluble iron salts with which the paper was coated. This reaction reinforces the effect produced by precipitating an insoluble ferricyanide in place of the soluble ferricyanide originally coated on the paper. If the ferric ion be bound in an insoluble compound at the same time that the ferricyanide ion is bound in an insoluble compound, the chance is reduced of the formation of either ferric ferrocyanide or ferrous ferricyanide-both of which are coloured-when the print is exposed in light.

The buffer mixture should maintain a sufficiently high acidity to permit the full development of the blue colour where the sensitized paper has been initially exposed to light. A suitable buffer reagent is provided by an ammonium or alkali salt of a weak acid, such as an ammonium or alkali acetate or citrate, together with an appropriate acidifler; but the preferred buffer reagent is a mixture of a monobasic and dibasic phosphate in approximately equal proportions with or without a small amount of an acid sulphate.

The following are examplesof suitable mix per cent of acetic for the developer.

The process is carried out to the best advantage with ferro-pruss'iate papers which do not contain, as is usually the case, an excess of ferric salt relatively to ferricyanide, because this excess causes a gradual fading of the background in the finished print; but a drawback to the use of such paper is that an excess of ferricyanide relatively to ferric salt results in slow printing, probably because the yellow colour of the ferricyanide inhibits full action of the blue actinic rays upon the salt of iron. An excess of ferricyanide carries also this disadvantage, that,.a large concentration of iron salt not being possible, the maximum depth of colour of the paper is not great.

The comparatively small depth of colour of a ferro-prussiate paper of the above type may be corrected in one way by applying to the surface a suitable colouring matter as nearly as possible complementary in colour to Prussian blue. For example, the developer may include a small quantity of a substantially complementary yellow dvestufi, whereby a print may be obtained which shows bright yellow lines upon a dark-green back ground.

A ferro-prussiate paper as aforesaid, wherein ferricyanide is definitely in excess of ferric salt, may be prepared by first coating or impregnating the base with the ferrlcyanide and then, after drying, with the ferric salt. A paper so prepared provides, notwithstanding the excess of ferricyanide, a fairly quick-printing medium. Alternatively, the iron and ferricyanide may be applied together, but the former process appears tobe the more advantageous.

For the most satisfactory results, especially as regards the stability of the print, it is desirable to use a paper of low content in sulphite-treated pulp; and a good class paper is to be recommended.

It is preferred, both for the manufacture of a ferro-prussiate paper and for the development thereof according to this invention, to apply the appropriate reagents to the base by a machine, desirably of that type which provides for the distribution of the reagent in a thin and uniform film over the surface of the base.

Example I A ferro-prussiate paper after exposure to light behind a design was sponged over by hand with a solution containing 10 grams of manganese sulphate in cc. of water, and was then dried.

Example II A ferro-prussiate paper after exposure to light behind a design was fixed in a coating machine carrying a-thin film of liquid over the surface of the paper from a trough charged with a solution containing 20 grams of nickel sulphate and 4 grams of potassium persulphate dissolved in 100 cc. of water.

Example III A ferro-prussiate paper after exposure to light behind a design was fixed in a machine as in Ex;

ample II by treatment with a solution containing grams of zinc chloride; 2 grams of potassium persulphate and 1 cc. of sulphuric acid dissolved I in 100 cc. of water.

Example IV A i'erro-prussiate paper after exposure to light behind a design was fixed in like manner in a machine by treatment with a developing solution of the composition: y Y Nickel sulphate 88 grams Ammonium persulphate 13 grams Ammonium dihydrogen phosphate--- 22 grams Diammonium hydrogen phosphateua 22 grams Sodium bisulphate 6 grams Sodium chlorate f 44 grams Water 440 cc.

This formula may be varied by replacing the nickel sulphate by a substantially equal weight of zinc or manganese sulphate. If applied by hand, for example, by a sponge, the solution should be diluted in the proportion of 3 parts by volume of additional water to every 10 parts by volume of the solution, as previously explained.

Example V A sheet of slightly absorbent, good quality ragpaper was treated in a coating machine with a solution composed of Potassium ferricyanide '7 ozs. Water 2. 4,000 cc.

and then, after drying, was treated in like manner with a solution composed of:

Ferric ammonium oxalate 3.5 ozs. Ammonium oxalate", 5 ozs. Water 4,000 cc.

Water, .100 cc.

Alternatively, the above quantity of ferricyanide (7 ozs.) may be dissolved in the ferric.

ammonium oxalate solution, and the paper then coated with this latter.

The invention eliminates the washing procedure of the usual ferro-prussiate process and provides a rapid and easy method of positive development of ferro-prusslate papers with production of an improved picture; and also permits of the conjoined use of a printing and a developing machine, so that finished prints may be made from a plan with increased facility and speed of operation.

I claim: 1. A process for the production of stable photographic prints consisting in exposing to light a layer of light sensitive material containing a soluble ferric salt and a soluble ferricyanide behind a transparency, and surface treating the exposed layer with an aqueous solution containing a chemical fixing agent for one of the prim'aryreagents of the layer inorder to effect a chemical transformation of the primary reagent present in the unexposed parts of the layer and so render the hind a transparency;

hind a pa n y, and surface treating the ex- 5 I posed layer with an aqueous solution containing I a salt of one of the metals of the group consisting of nickel, cadmium, manganese, zinc and cobalt, 10

. in order to eflect a chemical transformation'of the ferricyanide present in the unexposed part; of the layer and so render the unexposed parts of the layer non-sensitive to light.

3. A process for the production of stable photographic prints consisting in exposing to light a I layer of light sensitive material containing a soluble ferric salt and a soluble ferricyanide behind a transparency, and surface treating the exposed layer with an aqueous solution containing a weak alkali in order to effect a chemical transformation of the ferric salt in the unexposed parts of the layer and so render the unexposed parts of the layer non-sensitive to light.

4. A process for the production of stable photographic prints consisting in exposing to light a layer of light sensitive material containing a soluble ferric saltand a soluble ferricyanide behind a transparency, and surface treating the exposed layer with an aqueous solution containing orthophenanthroline in order to eiTect a chemical transformation of the ferric salt in the unexposed parts of the layer and so render the unexposed parts of the layer non-sensitive to light.

-5. A process for the production of stable photographic prints consisting in exposing to light a layer of light sensitive material containing a soluble-ferric salt and a soluble ferricyanide behind a transparency, and surface treating the exposed layer with an aqueous solution containing a chemical fixing agent for one of the primary reagents of the layer and an oxidizing agent in order to effect a chemical transformation of the primary reagent present in'the unexposed parts of the layer and so render the unexposed parts of the layer non-sensitive to light.

6. A proeess'for the production of stable photographic prints consisting in exposing to light a layer of light sensitive material containing a soluble ferriosalt and a soluble ferricyanide behind a transparency, and surface treating the exposed layer with an aqueous solution containing a chemical fixing agent 'for one of the primary reagents of the layerand'both a strong and a mild oxidizing agent in order to eifect a chemical transformation of the primary reagent present in the unexposed parts of e layer and so render the unexposed parts of the layer non-sensitive to light.

7. A process for the production of stable photo graphic prints consisting in exposing to lighta layer of light sensitive material containing a soluble ferric salt and a soluble ferricyanide beand surface treating the exposed layer with an aqueous solution containing a chemical fixing agent, for one of the primary reagents of the layer and a, buffer reagent in order to effect a chemical transformation of the primary reagent present in the unexposed parts of the layer and so render the unexposed parts of the layer non-sensitive to light.

8. A process for the production of stable photographic prints consisting in preparing a ferroprussiate paper by applying a solution of ferricyanideto the paper, permitting the'jsolution to chemical fixing agent for one of the primary reagents of the layer in order to effect a chemical transformation of the primary reagent present in the unexposed parts of the layer and so render the unexposed parts of the layer non-sensitive to light.

HUMJPHREY DESMOND MURRAY. 

